Method for pulse width modulation of a radar system

ABSTRACT

Radar systems are used for ascertaining the range to at least one reflecting object present in an observation area and/or for ascertaining the speed of the at least one reflecting object. Such a radar system emits into the observation area successive transmitter pulses having a determined pulse duration, with a determined pulse-repetition frequency as a transmitted signal. The system receives transmitted pulses reflected by the at least one reflecting object of the transmitted signal as a received signal. The disadvantage thereby is, that the method has high signal dynamics, which has an unfavorable effect on the price of the circuit components required for the signal processing. The new method for operating a radar system shall be performable with cost effective means.  
     In the new system the signal dynamics are reduced by ascertaining the mean power of the received signal and limiting it to a predetermined power range by varying the mean power of the transmitted signal preferably by variation of the pulse repetition frequency and/or the pulse duration of the transmitter pulses.  
     The method is well suited for operating a motor vehicle range warning system.

[0001] The invention relates to a method for operating a radar systemaccording to the preamble of the patent claim 1.

[0002] Such a method is known, for example, from DE 197 54 720 A1. Theknown method enables the simultaneous or separate determination of therange and/or the speed, particularly the relative speed, to one or morereflecting objects present in an observation area. For this purpose itis provided that the radar system is switched several times back andforth for short timed intervals between a transmitting operation and areceiving operation in a plurality of measuring phases of a measuringprocedure. Thereby, a pulse shaped transmitted signal is transmitted ineach measuring phase of the measuring procedure during the respectivetransmitting operation of at least one transmitter unit of the radarsystem. The pulse shaped transmitted signal includes a sequence oftransmitter pulses having a defined pulse duration and a determinedcarrier frequency. The transmitted pulses are emitted at a time intervalthat is determined by a pulse repetition rate. The transmitted pulsesare reflected by the reflection object or objects to the radar systemwhich receives the transmitted and reflected pulses and evaluates themas a received signal.

[0003] Hereby it is a disadvantage that the energy reflected by thereflecting object or by the reflecting objects, that is: the mean energyof the received signal, is strongly dependent on the range to thereflecting object or the reflecting objects. Thus, the method has highsignal dynamics which unfavorably affect the price of the circuitcomponents required for the signal processing.

[0004] Accordingly, it is the object of the invention to improve themethod according the preamble of patent claim 1 to the effect that itcan be performed with cost advantageous means.

[0005] The object is achieved by the features of patent claim 1.Advantageous embodiments and further developments are defined in thedependent claims.

[0006] In the method according to the invention for operating a radarsystem for ascertaining the range to at least one reflecting object thatis present in the observation area and/or the speed, particularly therelative speed of the at least one reflecting object, transmitter pulsesare emitted in a timed sequence into the observation area as atransmitted signal. The transmitter pulses have a defined pulse durationand a determined pulse repetition frequency. Transmitter pulses of thetransmitted signal that are reflected by the at least one reflectingobject, are received as received signals. Further, the mean power of thereceived signal is ascertained and limited to a predetermined powerrange by varying the mean power of the transmitted signal. Thereby, themean power of the transmitted signal is preferably varied by varying thepulse repetition rate and/or the pulse duration of the transmitterpulses.

[0007] According to an advantageous development of the method in casesin which the ascertained value of the mean power of the received signalis larger than an upper power value, the pulse repetition frequencyand/or the pulse duration of the transmitter pulses is variedcontinuously or in steps. The reducing is preferably performed at agiven speed for varying until the thereafter resulting mean power of thereceived signal is smaller than or equal to the upper power value.

[0008] In cases in which the ascertained value of the mean power of thereceived signal is smaller than a lower power value, the pulserepetition frequency and/or the pulse duration of the transmitter pulsesis advantageously increased continuously or in steps until thethereafter resulting mean power of the received signal is larger than orequal to the lower power value.

[0009] Preferably the transmitter pulses are emitted with a determinedcarrier frequency which is kept constant during the pulse duration ofthe respective transmitter pulse. According to an advantageous furtherdevelopment of the method the carrier frequency is successively variedin at least one measuring phase from transmitter pulse to transmitterpulse.

[0010] Furthermore, the range and/or the speed of the at least onereflecting object present in the reflection area, is advantageouslyascertained by evaluating the frequency difference and/or the phasedifference between the transmitted signal and the received signal.

[0011] A motor vehicle range warning system will be described in moredetail in the following as an example embodiment of the invention, withreference to the Figs., which show:

[0012]FIG. 1 a block diagram of the radar system,

[0013]FIGS. 2a to 2 c time diagrams of the transmitted and receivedsignals; and

[0014]FIG. 3 a schematic illustration of the frequency spectrum of thereceived signal converted, as to frequency, into the base band.

[0015] A radar system working as a motor vehicle range warning systemmust determine the distance and, if necessary also the speed,particularly the relative speed, of at least one reflecting object,clearly and with a high resolution. As a rule the determination must bemade simultaneously for all reflecting objects present in theobservation area, that is for vehicles ahead, oncoming vehicles andfollowing vehicles, persons and any other reflecting objects. Forexample, the desired definite range area is 150 m, the desiredresolution is 1 m and the desired speed resolution is 1 m/s. For thispurpose, an antenna emits a transmitted signal with a transmitterfrequency of, for example 76 GHz during at least one measuring phase ofthe measuring procedure. Following passage through the transmissionrange the antenna detects the reflection signal as an analog receivedsignal, which is received as a result of reflection by the vehiclesahead and following (reflecting objects). Thereby the same antenna isused for the transmitting operation and for the receiving operation.However, different antennas may be provided for sensing differentangular ranges at successive measuring operations. The received signalis further processed by a signal processing unit and evaluated withregard to any frequency shift and/or phase shift. Spectral analysis isapplied to the result to obtain the range information and, ifapplicable, the speed information.

[0016] According to FIG. 1 the radar system comprises, for this purpose,the following construction

[0017] a transmitter-receiver unit 1 with an antenna unit 11, atransmitter section 1 a, a receiver section 1 b, and an oscillator 13;the unit 1 combines the functions of the transmitter unit and of thereceiver unit, the essential components of the unit 1 are compactlycombined as a uniform module,

[0018] a signal processing unit 2 including a preamplifier 21 for bandwidth limiting, a digital signal processing unit 22 for exampleconstructed as a digital signal processor, a frequency converter 23, alocal oscillator 24, an anti-aliasing filter 25, and an analog todigital converter 26;

[0019] a control unit 3 which controls the two HF-switches ortransmitter receiver switch 121 and the LO-switch 122, the HF-switchingunit 12 and the modulation of the oscillator 13. The control unit 3 iscontrolled by the digital signal processing unit 22 for synchronizingthe transmitter-receiver unit 1 and the signal processing unit 2.

[0020] The antenna unit 11 of the transmitter-receiver unit 1 isprovided for emitting the transmitted signal and for detecting thereceived signal. The antenna unit comprises several antennas 111, 112,113 and an antenna switch 121 for selecting the respective antenna 111,or 112, or 113, whereby each of the antennas 111, 112, and 113 isselected for one respective measuring procedure having a respectivedifferent angular range of the transmitted signal. The switch-over ofthe antenna switch 114 takes place in response to the time duration ofone measuring procedure. A switch-over between the transmitter section 1a and the receiver section 1 b of the transmitter receiver unit 1 can beperformed by the HF-switching unit 12 with its both HF-switches ortransmitter-receiver switches 121 and LO-switch 122, that is, betweenthe transmitting operation and the receiving operation. Thetransmitter-receiver switch 121 and the LO-switch 122 are set in theleft position for the transmitting operation and in the right positionfor the receiving operation. HF-radiation is produced by an oscillator13, constructed for example as a VCO. Additionally the oscillator 13 canbe modulated linearly to switch the oscillation frequency in a linearfashion, for example with a switching frequency of 500 kHz, i.e., thecarrier frequency of the transmitter pulses can be successively variedwithin a predetermined frequency band width. Thereby, the carrierfrequency is kept constant during a transmitter pulse. During themeasuring phases of the measuring procedure the switch-over betweentransmitting operation and receiving operation is performed repeatedly.For example, if the pulse repetition frequency is 500 kHz, the periodduration tP of a pulse cycle including the pulse duration tON and thepulse pause is, for example, 2 μs. The pulse width repetition ratio,that is, the ratio between the pulse duration tON and the periodduration tP is adjusted to for example 50% at the beginning of themeasuring procedure. The receiver section 1 b of thetransmitter-receiver unit 1 detects as received signal the reflectedsignal caused by the latest emitted transmitter pulse, of all reflectingobjects prior to the emission of the next transmitter pulse. A mixer 14provided in the receiver section 1 b of the transmitter-receiver unit 1converts the received signal into a mixed signal in an intermediatefrequency band by multiplying the received signal with the oscillatorfrequency which is constant during a transmitter pulse.

[0021] The mixed signal generated by the mixer 14 is amplified by thepreamplifier 21 and simultaneously filtered. The preamplifier 21 limitsthe band width. In order to avoid coupling-in of disturbing signals itis suggested to deactivate the preamplifier 21 through the control unit3 during the transmitting operation. The amplified and filtered mixedsignal is then supplied to the frequency converter 23. The converter 23converts the amplified and filtered signal with an intermediatefrequency into the base band. The intermediate frequency is generated bythe local oscillator 24. The mixed signal transformed into the base bandis then passed through the anti-aliasing filter 25 to the analog todigital converter 23 which scans the signal and thus digitizes it. Theanti-aliasing filter 25 has a limit frequency of about 100 kHz, becausefrequency shifts between the transmitted signal and the received signalin the use of the system in motor vehicles, are smaller than 100 kHz andare thus in the pass-range of the anti-aliasing filter 25. The digitizedmixed signal is then processed by the digital signal processing unit 24by spectral analysis.

[0022] Alternatively, the received signal can be converted into the baseband already in the transmitter-receiver unit 1 by the mixer 14. In thatcase the second frequency conversion by the frequency converter 23 isnot necessary and the function of the anti-aliasing filter 25 can beperformed by the preamplifier 12 due to its band limiting effect.

[0023] The mean power of the received signal is ascertained in thedigital signal processing unit 24 on the basis of the amplitude of thedigitized mixed signal. A check is being made whether the ascertainedvalue of the mean power of the received signal is larger than apredetermined upper power value or whether it is smaller than apredetermined lower power value. In case the ascertained mean value islarger than the upper power value the mean transmitter power is reducedby reducing the pulse repetition frequency and/or the pulse duration ofthe transmitted pulses until the thereafter resulting mean power of thetransmitted signal is below the upper power value. On the other hand, ifthe ascertained value is smaller than the lower power value, the meantransmitter power is increased by increasing the pulse repetitionfrequency and/or the pulse duration until the thereafter resulting meanpower of the transmitted signal exceeds the lower power value. Thus, themean power of the received signal is adjusted to a value between theupper and the lower power value.

[0024] The relationship between the pulse repetition frequency and thepulse duration and the mean power of the received signal can best beexplained with reference to FIGS. 2a to 2 c. In these Figs. thetransmitted signal is designated as s, the received signal as r, thepulse duration of a transmitted signal as tON, the period duration as tPand the signal transit time is designated as tR0. Thereby, only theenvelope curves of the transmitted and received signals are shown. Thepulse repetition frequency 1/tP is selected, for example as follows: inthe time diagram of FIG. 2 a it is 500 kHz, in the time diagram of FIG.2b it is 250 kHz, and in the time diagram of FIG. 2c it is again 500kHz. In the case of FIGS. 2a and 2 b the reflecting object is so closeto the radar system that the signal transit time tR0 from the radarsystem to the reflecting object and back is shorter than the pulseduration tON. The radar system is capable of receiving only startingwith the point of time tON. Therefore, the area illustrated withhatching lines and representing reflected pulses is cut out. The meanpower of the received signal r is equal to the surface area under thereceived pulse referenced to the period duration tP. Therefore, it ispossible to directly ascertain from FIGS. 2a to 2 c that the mean powerof the received signal r in the case of FIGS. 2b and 2 c is smaller thanin the case of FIG. 2a, i.e. the mean power of the received signal r isreduced by increasing the period duration tP, that is, by reducing thepulse repetition frequency, and by reducing the pulse duration tON ofthe transmitted signal s.

[0025] Due to the relative motion between the reflecting object and theradar system one obtains a frequency shift between the carrier frequencyof the transmitted signal s and the carrier frequency of the receivedsignal r as seen in FIG. 3. FIG. 3 shows the frequency spectrum of thereceived signal r according to FIG. 2a after its conversion into thebase band, i.e. of the signal supplied to the anti-aliasing filter 25.The line designated with r0 thereby corresponds to the main line of thereceived signal r. The position of the line r0 is a measure of thefrequency shift between the carrier frequency of the transmitted signals and of the carrier frequency of the received signal r.

[0026] The frequency shift is ascertained by the next following digitalsignal processing stage 22. The secondary lines of the received signal rare designated as r1 and r1*. FIG. 3 also shows the frequency range h0to be evaluated and the pass range h of the anti-aliasing filter 25. Thesecondary lines r1, r1* are positioned outside of the pass range h andare thus suppressed by the anti-aliasing filter 25. If the pulserepetition frequency is reduced to one half for example from 500 kHz,for reducing the main power of the transmitted signal s, the spacingbetween the spectral lines r0, r1, and r1* is also reduced to one half.The secondary lines r1, r1* continue to remain, thereby, outside thepass range h and are thus still suppressed by the anti-aliasing filter25. Additionally, the spectral lines are also reduced in their heightwhich corresponds to the desired reduction of the mean power of thereceived signal r.

[0027] On the other hand, if starting with a transmitted signal saccording to FIG. 2a the pulse duration tON of the transmitted pulses isreduced as shown in FIG. 2c, such reduction has no influence on theposition of the spectral lines r0, r1, and r1* of the received signal rprovided the pulse repetition frequency tP⁻¹ remains constant.

[0028] Due to the reduction of the signal dynamics it is possible to useanalog to digital converters and digital signal processing units whichprocess signals having small bit widths and hence can be produced costefficiently.

[0029] The method according to the invention is not limited to its usein the radar system of FIG. 1. Rather, it is usable in any radar systemfor motor vehicles, which emits transmitter pulses with a determinedcarrier frequency, which receives the transmitted and reflected pulsesas a received signal, and which limits the band width prior toevaluation. The evaluation of the received signal with regard to thepower limitation of the transmitted signal can thereby be performed inthe same-signal processing branch in which the range and/or the speed ofthe reflecting object is ascertained. However, the evaluation can alsobe performed in a separate circuit section provided especially for thepower limitation.

1. A method for operating a radar system for ascertaining the range toat least one reflecting object present in an observation area, and/orfor ascertaining the speed of the at least one reflecting object,wherein the radar system emits in a timed sequence and with a determinedpulse repetition frequency (tP⁻¹) transmitter pulses having a determinedpulse duration (tON) as a transmitted signal (s) into the observationarea, and which receives transmitted pulses of the transmitted signal(s), which are reflected by the at least one reflecting object, asreceived signals, characterized in that the mean power of the receivedsignal (r) is ascertained and limited to a predetermined power range byvarying the mean power of the transmitted signal (s).
 2. The method ofclaim 1, characterized in that, in case the mean power of the receivedsignal (r) exceeds an upper power value, the mean power of thetransmitted signal (s) is reduced by reducing the pulse repetitionfrequency (tP⁻¹) and/or the pulse duration (tON) of the transmitterpulses until the mean power of the received signal (r) is smaller thanor equal to the upper power value.
 3. The method of claim 2,characterized in that, in case the mean power of the received signal (r)falls below a lower power value, the mean power of the transmittedsignal (s) is increased by increasing the pulse repetition frequency(tP⁻¹) and/or the pulse duration of the transmitter pulses until themean power of the received signal (r) is larger than or equal to thelower power value.
 4. The method of one of the preceding claims,characterized in that the transmitted pulses are respectivelytransmitted with a determined carrier frequency that is constant duringthe pulse duration (tON).
 5. The method of claim 4, characterized inthat in at least one measuring phase the carrier frequency issuccessively varied from one transmitted pulse to the next transmittedpulse.
 6. The method of one of the preceding claims, characterized inthat the range and/or the speed of the at least one reflecting object isascertained by evaluating the frequency difference and/or the phasedifference between the transmitted signal (s) and the received signal(r).